Atomic sensors comprise for example atomic clocks, micro-magnetometers, or micro-gyros. Such atomic sensors may for example be for telecommunication, navigation, and defense systems.
Such optical gas cells are sometimes called “cells” or “microcells”, and the associated sensors “micro-atomic clocks”, “micro-magnetometers”, and “micro-gyros.” Throughout the text, the term “micro” is to be understood within the context and with the meaning indicated.
A typical application is a chip-scale atomic clock, known by the acronym CSAC.
The operation of atomic sensors is based on optical spectroscopy of atoms of a gas filling the cavity of a cell provided with at least one optical window. This cavity is thus described as an “optical cavity”. The gas is usually an alkali vapor, such as cesium or rubidium. This spectroscopy enables measuring one or more spectral values associated with the physical quantity or quantities which the sensor is to observe, for example a frequency, a magnetic field, or an acceleration.
For example in the case of a micro-atomic clock, the operation of the sensor can be based on measuring the frequency of a particular microwave transition of the gas atoms, called the clock transition. In this application, the micro-atomic clock then typically implements the principle of atomic resonance by coherent population trapping, known by the acronym CPT.
Furthermore, the width of the line observed by spectroscopy, and thus the frequency stability in the case of an atomic clock, is determined by the relaxation time of the alkali atoms starting from the coherent state in which they were pumped to their ground state, a time which is primarily dependent on atom collisions with the cell walls, resulting in loss of coherence.
To improve the quality of the spectroscopy of the gas, and thus the accuracy and stability of the atomic sensor, it is well known to add a gas to the alkali vapor, referred to as buffer gas or buffer atmosphere, which slows the diffusion of alkali atoms toward the cell walls and confines said alkali atoms.
Such atomic sensors offer the advantage of being small in size and energy efficient, and of having very good measurement precision and stability.
An example of such an atomic sensor is known from the work of the MAC-TFC consortium, in which the FEMTO-ST Institute (acronym for Franche-Comté Electronique Mécanique Thermique et Optique-Sciences et Technologies) has designed and built a highly compact cesium vapor cell (a few mm3) for an atomic clock, with MEMS (acronym for MicroElectroMechanical Systems) micromachining of the silicon and anodic bonding (for example see “New approach of fabrication and dispensing of micromachined cesium vapor cell” by L, Nieradko, C. Gorecki, A. Douahi, V. Giordano, J.C. Beugnot, J. Dziuban, and M. Moraja published in JOURNAL OF MICRO-NANOLITHOGRAPHY MEMS AND MOEMS of August 2008).
The production of optical gas cells and of such atomic sensors usually relies on methods that microfabricate stacks of silicon and glass substrates fixed together by anodic bonding.
A usual cell microfabrication method begins by etching cavities in a silicon substrate. A first glass substrate is then welded, usually by anodic bonding, on one side of this substrate. Finally, a second glass substrate is welded to the opposite side after incorporation of the buffer atmosphere and the alkali metal, which may take various forms depending on the filling method used.
To introduce the alkali metal into the optical cavity, the fabrication may be carried out, particularly the step of anodic bonding, in an atmosphere containing cesium and a buffer gas or by depositing a certain amount of alkali metal in liquid or solid form into a cell cavity.
However, there are complications when sealing in the presence of pure cesium. The anodic bonding must start at a low temperature, to avoid evaporation of the metal deposited in the cavity, and continues as the temperature is increased. This can lead to pressure differences in the buffer gas as indicated in document US2012/0298295A1.
To simplify fabrication of the atomic sensor and to allow using standard anodic bonding equipment in optimum conditions, it is known to use a solid compound called a dispenser that is introduced into the cell during the sealing step. Such a compound is for example made of an alloy of Zr—Al and cesium chromate, suitable for remaining stable at the temperature of the anodic bonding. The dispenser is then locally heated, for example by means of a high-power laser, to release pare cesium and create saturated vapor in the cell. After activation, most of the cesium atoms are in liquid phase or solid phase depending on the temperature. Such a method is detailed for example in the document “From the Implementation to the Characterisation and Assembling of Microfabricated Optical Alkali Vapor Cell for MEMS Atomic Clocks” by Nieradko, Lukasz, et al., published in 2007 in the Solid-State Sensors journal, pages 45-48.
Although it simplifies fabrication of the sensor, the presence of a dispenser in each cell is also a constraint. It limits the density of cells that can be achieved on a wafer, which increases the production cost per cell. It also increases the size of the cell, and therefore that of the atomic sensor in which it is integrated. The cell also suffers from greater thermal dissipation (larger radiating surface). Furthermore, the use of one dispenser per cell imposes a substantial fixed cost for each cell. The amount of cesium released during the heat activation is also difficult to control. The amount of alkali metal can thus vary significantly between cells in the same batch. There can then be excessive condensation of cesium which can obstruct the optical window, or conversely the amount of cesium may be insufficient to ensure a satisfactory service life. Furthermore, a study has indicated that the dispenser could cause variations in the atmosphere and compromise the performance of the clock in which it is integrated (“Aging Study on a Micro-Fabricated Cs Buffer-Gas Cell for Atomic Clock Applications” by Abdullah, Salman et al. published in 2014 in Proc. European Frequency and Time Forum). Finally, the dispenser is usually not fixed in the cavity, and can strike the walls if the cell is subjected to impacts or vibration. The component particles of the dispenser can then break up and obstruct the optical window.